1. Field of the Invention
The present invention relates to a switchable permanent magnetic chuck, and more particularly to a thin-type permanent magnetic chuck.
2. Description of the Prior Art
A typical conventional chuck of this type is disclosed, for instance, in French Patent Application No. 7118322. The permanent magnetic chuck disclosed in this French Patent Application comprises a shiftable plate which is accomodated in a frame structure and movable in the direction of the arrangement of magnetic pole face portions of a work supporting faceplate attached to the frame structure. On this shiftable plate, pole pieces made of soft iron and permanent magnets are alternately arranged with an equal pitch in the direction of its shifting movement, with the magnetization directions of the magnets being directed either upwardly or downwardly to the same direction. Likewise, on the frame structure, pole pieces made of soft iron and permanent magnets are alternately arranged, along the shiftable plate, with the same pitch as the pitch of the pole pieces and the permanent magnets of the shiftable plate, with the magnetization directions of the magnets of the frame structure being directed in the same direction as the magnetization directions of the permanent magnets of the shiftable plate.
When the movable permanent magnets of the shiftable plate are brought in transverse alignment with the stationary permanent magnets of the frame structure, the magnetic pole face portions of the work supporting faceplate will be magnetized to establish alternately inverted magnetic poles along the direction of their arrangement, whereby the work supporting plate will be brought in a magnetized condition. In this case, the movable pole pieces provided on the shiftable plate and the stationary pole pieces provided on the frame structure will establish, together with the adjacent movable permanent magnets and stationary permanent magnets, respectively, magnetic paths.
On the other hand, when the shiftable plate is moved to bring the movable permanent magnets thereof in aligment transversely with the stationary pole pieces of the frame structure, whereby the movable pole pieces of the shiftable plate will be brought in alignment transversely with the stationary permanent magnets of the frame structure, the transversely aligned pairs of the permanent magnets and pole pieces will establish magnetic paths, whereby the work supporting plate will be brought in a demagnetized condition.
As is apparent from the foregoing, the pole pieces establish, together with the respective permanent magnets, closed magnetic paths irrespective of whether the work supporting plate is in the magnetized condition or in the demagnetized condition. It is accordingly conceivable that by replacing the pole pieces by permanent magnets magnetized inversely of the magnetization directions of the afore-mentioned permanent magnets, it is possible to reduce the height of the respective permanent magnets to a half without reducing the overall magnetic power.
However, in the conventional permanent magnetic chuck, alnico magnets having a low coercive force were used as the permanent magnets, and it was impossible to replace the pole pieces by such permanent magnets. Namely, because of their low coercive force, it was necessary that the alnico magnets were assembled in the state of non-magnetized ferromagnetic material together with the pole pieces into the shiftable plate and the frame structure, and after the work supporting faceplate, etc. were further attached thereto to complete the assembly for the magnetic circuit of a chuck, an external magnetic field was applied to the ferromagnetic material to magnetize the ferromagnetic material to permanent magnets. Accordingly, if the pole pieces were to be replaced by the permanent magnets, it would be necessary to magnetize a number of such ferromagnetic pieces in alternately inversed directions along the direction of their arrangement, in the magnetization operation as mentioned above. Such a magnetization operation is practically very difficult, if not impossible.
Accordingly, in the conventional permanent magnetic chuck, the use of the pole pieces was essential and it was necessary to use permanent magnets having a relatively great height to avoid a reduction of the magnetic power, whereby it was impossible to reduce the overall height of the chuck.